This invention relates to methods for automatically limiting distortion in audio devices and to circuit arrangements for carrying out such methods.
Because of the great nonlinearity of the transfer characteristic of amplifiers, distortion occurs when the amplitude of an input signal is not infinitesimal. A measure of the amount of distortion is the harmonic distortion factor. The harmonic distortion factor indicates the ratio of the root-mean-square values of the harmonics and the fundamental at the output.
In car radios, in particular, the volume is frequently set so high that the output stages are overdriven and, consequently, a high harmonic distortion factor is produced. This can also occur, for example, when a switch-over is made from a signal having a relatively low frequency swing to a signal having a high frequency swing, after the volume had previously been set to an acceptable value with regard to the harmonic distortion factor. In order to avoid an excessively large harmonic distortion factor, therefore, a fixed threshold value between 0.2% and 20% harmonic distortion factor is set, and if this limit value range is exceeded, the gain of the audio device is reduced until the harmonic distortion factor again falls below the limit value of the harmonic distortion factor.
Such an apparatus is disclosed, for example, in German Offenlegungsschrifft No. 43 40 167, in which it is possible to detect at the output stage of the audio device a signal which indicates a predetermined degree of driving. The detected signal, which is passed through an integration element, provides a voltage representing the harmonic distortion factor and, based on a predetermined characteristic curve, produces a manipulated variable which is transmitted to a final control unit for the gain of the audio signals. This arrangement has the disadvantage that, when bass bursts occur, the harmonic distortion factor is exceeded and the output power of the audio signal is reduced over its entire frequency range. This results in so-called ",breathing", that is to say the overall audible volume of the audio signal varies with the rhythm of the bass bursts that occur.
U.S. Pat. No. 5,255,324 discloses an arrangement for adapting the distortion in audio devices in which the bass range is selectively observable and controllable in order to avoid "breathing". The principle is explained in more detail with reference to FIG. 2 of that patent. A fixed, frequency-independent limit value is defined for the harmonic distortion factor. If, at any signal frequency, the output signal of the audio device exceeds the limit value of the harmonic distortion factor, then a check is first made to see whether the limit value of the harmonic distortion factor in the bass range has been exceeded. If this is the case, then only the bass gain is reduced and a renewed check is made to see whether the output signal lies above the limit value. If this is the case, then a check is made to see whether the limit value of the bass range is still being exceeded. Only when the limit value of the bass range is no longer being exceeded yet the output signal still exceeds the limit value the harmonic distortion factor is the gain of the entire frequency range reduced. This arrangement has the disadvantage of complicated and slow regulation.